Resistance microphone and method of operating the same



Aug. 28, 1928. 1,682,443

P. THOMAS RESISTANCE MICROPHONE AND METHOD OF OPERATING THE SAME Filed June 16, 1922 WITNESSES: INVENTOR p/UW/ DS Thomas Patented Aug. 28 1928.

uN TEo sTArEs PATENT OF FICE.

PHILLIPS crnom s, 0P nnenwoon, PENNSYLVANIA, ASSIGNOR TO .wnsrmenousn nnnc'rnrc & mimuriicrumne COMPANY, A CORPORATION or PENNSYLYANIA.

RESISTANCE MICROPHONE AND nnrnon or oPnRA'rI'Ne rim sun Application filed June 1c, 1922. Serial No. 568,689,

invention relates to telephone systems and apparatus and particularly'to an improved "circuit arrangement for telephone transmitters or receivers of the resistancemicrophone type.

An object of my invention is to provide a sound-translating device and-circuit connections therefor which insure the translation of sound waves into correspondingly modulated currents Without the introduction of distorting harmonics, regardless of how hard I the device is worked,

Another object of my invention is toprovide a microphone and circuit connections therefor which, when once adjusted, maintain the same average adjustment over long periods of time.

Still another object of my invention is to provide a telephone system wherein the microphone transformermay be dispensed with, hence eliminating distortion caused bythe different impedances of the transformer for dilferent applied frequencies.

A'further object of my invention is to provide such atelephone system as to render unnecessary the careful selectioi'fof a microphone therefor. I r

In the usual variable-current telephone system employing a transmitter of the resistance type, the microphone is connected 111 series with the primary or low-tension Wll'ldi ing of a transformer and also with asOurce of direct-current" energy of relatively lo w voltage. The major part of the resistance in such circuit isfurnished by the microphone itself and hence the voltage at the terminal 'of the microphone is substantially the constant voltage of the source.

In'the operation of such variable-current apparatus, the variations in the resistance of,

the microphone produce corresponding variations in the curren't flowing through the priniarywinding of the transformer which, in turn, produce alternating voltages inthe secondary winding thereof adapted-to actu- 4 ate a telephone receiver, either directly or through auxiliary a paratus.-

The sound imp'u ses impingin on the sound receiving mechanism of a microphone may be resolved, by the Fourier series, into fundamental sine. waves and a number of harmonics, and hence, the resistance of the microphone comprises a constant component,

' namely the quiet resistance of the microvphone, and a variable component which is rent may, therefore,

made up of substantially sinusoidalcomponfints corresponding, waves of the sound impulses.

Inthe previously used circuit arrange ments, wherein the microphone resistance is substantially the total resistance in circuit with a battery. or other constant source, the current is proportional to thereciprocallof' ,the microphoneir esistance, andlience, themore or less closely, to e component fundamental and harmonic components of current corresponding'to the various soundwave components are not sinusoidal but have a wave-form corresponding.

to the reciprocal of a constant plus a sine function. The variable portion of the cur D be resolved, by the Fourier series, into a number of sinusoidal components which include not only currents of frequency the same as that of the respective sound components but also currents of other frequencies corresponding to the variof the sound com- 'sulted inevitably iii the sacrifice of clearness,

inasmuch as the cl'iar acter or timbre of sound depends entirely upon the magnitude and the frequency of the overtones present. Such distortion, while endurable in speech modulation, causes acute discomfort when it occurs in the transmission and reception of music.

It is also true that, although the performance of a microphone of the carbon-capsule or resistance type isnot very definite,- that is, the ohmic resistance of the capsule does not always vary between certain fixed limits during exactly repeatable cycles, this desirable working conditionis most nearly approximated when the microphone is worked hard. Asijust brought out,'this is theworst condition for the faithful reproduction of sound waves in the ordinary variable-current systems.

According to my invention, I provide an improved circuit arrangement for a microphone transmitter of the resistance t e, whereby, the above-mentioned undesirzgle characteristics are avoided, Jllhe desired resalt is accomplished by employing a circuit arrangement which utilizes the variations in voltage across a microphone transmitter when the current through it is held constant, and, as the voltage is then directly proportional to the resistance, the introduction of the above-described current harmonics is avoided, and the instrument may, therefore, be operated at its most advantageous load.

Very briefly, this mav be accomplished by connecting the microphone in series-circuit relation with a very high resistance compared to that of the instrument itself and applying to the combination a source of direct-current energy having a voltage of sullicient value to produce the same current through the microphone as was formerly produced by the low voltage applied without the additional resistance. The quiet voltage at the microphone terminals will then be the same as before. Instead of employing a transformer for making the resistance variation available as a voltage variation, I employ the actual voltage variation at the terminals of the microphone and cause the same to be translated into correspondingly modulated currents by impressing the same upon the input electrode of a vacuum tube through a condenser having a negligibly low reactance to currents of sound-wave frequency.

IVith these and other objects and applications in view my invention further consists in the combinations and circuit arrangements hereinafter described, claimed and illustrated in the accompanying drawing, wherein the single figure is a diagrammaticview of circuits and apparatus embodying my invention.

In the illustrated embodiment of my in vention, a microphone transmitter 1 of the variable-resistance type has its terminals 2 and 3 connected to the plus and minus terminals 4 and 5 of a source 6 of direct-current energy by means of conductors? and 8, respectively. A resistor 9 is connected in series circuit relation with the conductor 7. The resistance of the resistor 9 is of such value that variations in the resistance of the microphone 1, incident'to the presence of sound waves, causes substantially no appreciable change in the intensity of the current traversing the same. In practice, I have found that the resistance of the resistor 9 may be at least ten times the so-called quiet resistance of the microphone l. The voltage of the direct-current source of energy 6 is of such value as to cause normal currents, say .25 ampere, to traverse the microphone 1.

The variable potentials effective across the microphone terminals 2 and 3, incident to the presence of sound waves, may be impressed, through conductors l5 and 16, respectively, upon the grid-filament elements 11 and 12 of a three-electrode vacuum tube 13 comprising, in addition, a plate 11.

A grid-leak resistor 17a may be connected in shunt relation to the grid-filament elements of the tube 13 to control the intensity of the negative charges built up on the grid 11 during the operation of the tube. A condenser 17, having an impedance to the lowest frequency of the sound waves which is very small compared to that of the grid-leak resistor 17a, is included in the conductor 15. As can readily be seen, the condenser 17 serves to prevent the so-called quiet, voltage drop across the microphone 1 from being impressed across the grid-filament elements of the tube 13 and hence it prevents the blocking of the tube. With the capacity of the condenser 17 relatively large, as just stated, substantially no distortion is introduced by the use of the same.

The hot cathode 12 may be energized by a source 18 of direct-current energy through a rheostat 19. A plate-filament circuit 21 for the tube 13 includes a source 22 of direct-current energy and a coupling coil 23. An outgoing or work circuit 24 1s operatively connected to the coupling coil 23 through a coupling coil 25. The work circuit 22 may, in turn, be connected to any circuit arrangement designed to employ modulated currents.

Any possible danger due to the presence of the somewhat high voltages, of say 100 or 200 volts, employed in the source (5 of direct-current energy, may be obviated by connecting the negative terminal 3 of the microphone to ground through a ground conductor 26.

In operation, when the resistance of the microphone 1 is varied by reason of the presence of sound waves, the potential drop thereacross is correspondingly varied, while the current therethrough remains substantially Ian constant, for reasons hereinbefore mentioned. -loo The variable potentials are impressed upon the input electrodes 11 and 12 of the tube 13 causing currents of corresponding modulation frequency to traverse the plate-filament circuit 21, as will readily be understood by those versed in the art. As before stated, the modulated currents may be further impressed on a work circuit 24 which is associated with other circuits and apparatus (not shown).

In addition to the several advantages hereinbefore mentioned, a further advantage of my invention resides in the fact that the microphone 1 may be arranged to receive a very loud signal from a source which it is desired to copy and hence will be very insensitive to so-called side tones.

Furthermore, I have found, experimentally, that certain types of microphones, which operated poorly in the usual telephone systems, have operated with a much greater degree of sensitivity in the system embodying my invention. Other advantages will be evident to those versed in the art.

XVhile I have shown only one embodiment. of my invention, for the purpose of describ lltI ing the same and more fully illustrating the principles thereof, it is capable of various changes and modifications in the operation and circuit arrangements without departing from the spirit of my invention, and I desire, therefore, that only such limitations shall be made thereon as are imposed by the prior art or specifically set forth in the appended claims.

I claim as my invention:

1 In an electrical system, a circuit comprising a resistance microphone, a resistor and means for supplying said microphone Withnormal energizing currents, said resistor being of such value as to cause the current flow in said circuit to remain substantiallyconstant, independent of variations-in the resistance of said microphone incident to the presence of disturbing impulses, a thermionlc device having grid, filament and plate elements, conductors connecting said grid and filament elements to opposite sides of said microphone, a condenser connected in one of vsaid leads having a negligibly low reacta nce to "currents of the lowest frequency to WhlCh said microphone is intended to respond, anda grid-leak circuit connected in shunt to said grid and filament elements.

2. In a modulating system, a source of energy having positiveand negative terminals, a microphone having terminal connections, means connecting said negative and, positive terminals to opposite microphone terminals, an impedance device connected in series circuit relation with one of said connecting means, said device having such value as to maintain the currents traversing said microphone substantially constant for variations in the resistance of said microphone, means for connecting the other of said connecting means to ground and means including a potential-responsive device for translating otential variations across said microphone incident to variations in the resistance there of into correspondingly modulated currents.

3, In a modulating system, a source of substantially constant-voltage having positive and negatiie terminals, a microphone having terminal connections, means connecting said negative and positive terminals to opposite microphone terminals, a resistor connected in series circuit relation with one of said connecting means, said resistor having such value as to maintain the currents traversing said microphone substantially constant for variations in the resistance of said microphone, athrec-electrode tube having grid, filament and plate elements, means connecting said grid and filament elements to opposite microphone terminal connections, a relatively large condenser included in one of said last-mentioned connecting means and grid-leak means connected in shunt relation to said grid and filament elements. a

4. In a modulating system, a, source of energy having positive and negative terminals, amicrophone having terminal connections, means connecting said negative and positive terminals to opposite microphone terminals, a resistor connected in series-circuit relation with one of said connecting means, said resistor having such value as to maintain the currents traversing said microphone substantially constant for variations in the resistance of said microphone, a three-electrode tube having grid, filament and plate elements, means connecting said grid and filament elements to opposite microphone terminal connections, a ca acitive reactance device included in said gri -1nicroph0ne connection and grid-leak means connected in shunt relation to said grid and filament element-s.

5.-The combination with a microphone transmitter of the resistance type, a resistance in circuit therewith so large that the microphone resistance is negligibly small in comparison, a direct-current source of substantially constant voltage therefor and potentialresponsive energy-amplifying means controlled by the resultant voltage-variations in said circuit.

In testimony whereof, I have hereunto subscribed my name this 12th day of June, 1922.

l PHILLIPS THOMAS. 

